Sky News meteorologist Kirsty McCabe explains: "The supercooled rain hits the ground and freezes instantly on impact, and that creates a thin layer of ice, also known as glaze, and it's clear, so you can't see the ice, which makes it really treacherous.
Freezing rain, which makes up what are commonly known as ice storms in North America, is a rarity in the UK because the conditions for it are quite specific, according to the Met Office.
It starts as snow, ice, sleet or hail high up in the atmosphere, but as it travels down it melts through a layer of warmer air, then refreezes again through a layer of colder air near the surface.
An amber warning for snow and ice, with the risk of freezing rain, covers most of Wales and central England until midday on Sunday.
The ice is very clear, often referred to as black ice, because it is so difficult to see, making it treacherous for pedestrians and drivers.
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